1. Field of the Invention
The present invention relates to a defect detector circuit and a magnetic disk certifier using the same defect detector circuit and, particularly, the present invention relates to an improvement of a magnetic disk certifier for testing a magnetic disk memory medium on error of electric characteristics thereof, such as missing error, spike error and/or thermal asperity error, to make the magnetic disk certifier possible to perform a defect test of the magnetic disk written with test data at high write frequency by using low frequency suitable for the defect detection.
2. Description of the Related Art
When a magnetic disk such as hard magnetic disk for recording an information is defective, a problem occurs in a recording performance of the hard magnetic disk. Therefore, the quality of the hard magnetic disk is evaluated or certified by performing a test on whether or not the magnetic disk has any electric defect of the disk and, if any, on the degree of the defect by a magnetic disk certifier.
In a conventional magnetic disk certifier, a test data having a predetermined code, for example, FFh, is generated according to a test signal having frequency f0 generated by a test signal generator circuit, is converted into a write current having a predetermined level by a write/read amplifier and is written in respective tracks of the magnetic disk sequentially through a magnetic head. Incidentally, xe2x80x9chxe2x80x9d of FFh indicates hexadecimal notation and FF indicates data having bits all of which are xe2x80x9c1xe2x80x9d.
When the write of the test data with respect to all of the tracks of the magnetic disk is completed, the written test data is read out sequentially as a positive phase read-out signal and a negative phase read-out signal by a magnetic head through a write/read amplifier and inputted to a level regulation amplifier. The two read-out signals having levels regulated by the level regulation amplifier are inputted to a slice level setting circuit and a defect detector circuit. In the slice setting circuit, a predetermined slice level for the read-out signals are generated and the slice level is inputted to the defect detector circuit. Incidentally, when the signal code of the test data is FFh, the frequency of each read-out signal becomes a half of a frequency f0 of the test data. In the defect detector circuit, defect detection is performed correspondingly to the slice level set by the slice level setting circuit.
Incidentally, the read-out signals have positive peaks and negative peaks with respect to a predetermined reference amplitude level, respectively.
With the recent increase of memory capacity of magnetic disk, the test frequency of a magnetic disk certifier, that is, the frequency of a test signal, is increased up to several hundreds MHz or higher. Therefore, it is necessary to use a processing circuit having high performance corresponding to such high frequency test signal as a defect detector circuit. In such case, the higher the test signal frequency is the more severe the defect detector circuit times a setting period required to detect a defect. Therefore, possibility of erroneous detection may become high. On the other hand, it is usual that such defect detector circuit is designed as a LSI circuit. However, the current circuit design for preventing erroneous detection when such high test frequency is used is not always satisfactory.
In order to solve these problems of the conventional magnetic disk certifier, U.S. Pat. No. 5,812,560, which is assigned to the assignee of the present application, discloses a technique, in which an error detection from a read-out signal is performed with using a thinned timing so that a defect detector circuit similar to the conventional magnetic disk certifier can be used by thinning the detection number of defects.
Since, in such thinning process, however, the error detection timing, that is, the bit period of the test data, is limited to a value which is one n-th the bit period of the test data to be detected, where n is an integer, there may a case where a defect can not be detected by selecting a suitable frequency.
An object of the present invention is to provide a defect detector circuit capable of detecting defect of a magnetic disk written with a test data at high frequency by using low frequency suitable for a defect detection.
Another object of the present invention is to provide a magnetic disk certifier capable of detecting defect of a magnetic disk written with a test data at high frequency by using low frequency suitable for a defect detection.
In order to achieve these objects, a defect detector circuit and a magnetic disk certifier for detecting an error of a magnetic disk on a basis of a read-out signal, which is acquired by writing a predetermined test data in a magnetic disk according to a test signal having specific frequency and reading the test data as having a frequency related to the frequency of the test signal, are featured by comprising a low pass filter for deriving a low frequency component containing a signal component of an error mainly from the read-out signal, a signal generator circuit for generating a signal having a predetermined frequency lower than the frequency of the read-out signal, a synthesizer circuit for synthesizing the signal outputted from the low pass filter with the signal outputted from the signal generator circuit and a comparator for comparing a synthesized signal outputted from the synthesizer circuit with a predetermined level to generate an error signal.
It has been known that, when defect of a magnetic disk is considered in bit unit of the test signal, the higher the recording density of the magnetic disk is the smaller the detection unit of defect of the magnetic disk. The size of defect, which is detectable by using one bit, is increased to some extent correspondingly to an increase of the recording density. Therefore, defect is usually detected as a plurality of bit defects when the recording density becomes high.
In other words, a defect detected by every write bit when a test data is written at high density is usually not one bit but a plurality of successive bits. In the defect detector circuit and the magnetic disk certifier according to the present invention having the above mentioned construction, the error frequency component, which is low, of the read signal and the basic frequency component thereof, which is high, are separated from each other. The high frequency of the basic frequency component thus separated is converted to a component having a lower frequency and synthesized with the error frequency component to generate a pseudo read signal. The defect detection is performed by this pseudo read signal having low frequency. The low frequency, which can be selected, is not limited to one n-th where n is an integer, contrarily to the case where the defect detection is performed by the thinning process of the error detection timing. According to the present invention, it is possible to select an optimal frequency in concord with the accuracy of defect detection.
As a result, the error detection of a disk tested at high frequency becomes possible at low frequency, so that the defect detection circuit, the setting of the detection timing of which is not severe and which hardly produces erroneous detection, is easily realized. Further, it is possible to realize the magnetic disk certifier having high reliability, which is not substantially lowered even when the defect detection is performed by using lower frequency than the test signal frequency.